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sanding fiberglass on canoe

I'm sanding the fiberglass/epoxy on the outside of my canoe. I started with 60 grit to help with some of the bigger runs then switched to 80. When going through with the 80 there is a couple of spots were I think I exposed the fiberglass cloth. I have only done 1 side so far with the 80 as I don't want to make the problem worse. Did I sand to far down to the cloth? If so what should I do next? Do I need to add more resin and how would I blend that into the rest of the canoe? My next step was going to be to wet sand with 320.

Re: sanding fiberglass on canoe

First of all, you don't go from 80 to 320. You would probably want something like 120-150 next and follow that with maybe 180 or 220. I don't think I ever sanded a stripper hull finer than 220 before varnish or paint. Making it too smooth does not make for good coating adhesion and 320 is not going to remove the scratches produced by the 80. There is often a certain amount of sanding into the glass if you happen to be tapering out extra layers of reinforcement, but you do want to avoid it as much as possible in other areas. My impression is that the whole thing could have used an additional filler coat or two. In the top photo you have the dark spots, where you have yet to reach the bottom of the crater with the sander, combined with spots which have already been sanded too deeply and well down into the cloth - all within an inch or two of each other.

If you want to see what cosmetic effect the oversanding will have, you can wipe the area down with a wet sponge. Most of the bad stuff will likely disappear when wet and will do the same once varnished. Strength-wise, it will depend on just how far down into the cloth the sander went in the various spots, but since you aren't likely to contact rocks with every square inch of the hull, you can probably live with the reduced cloth strength. I certainly might be tempted to add one more filler coat before doing any more sanding, and any time you're sanding and start to see a pattern of tiny, evenly-spaced whitish dots showing up, stop sanding that area. That is the top of the weave and further sanding will be cutting into the cloth.

Re: sanding fiberglass on canoe

Originally Posted by Todd Bradshaw

First of all, you don't go from 80 to 320. You would probably want something like 120-150 next and follow that with maybe 180 or 220. I don't think I ever sanded a stripper hull finer than 220 before varnish or paint. Making it too smooth does not make for good coating adhesion and 320 is not going to remove the scratches produced by the 80. There is often a certain amount of sanding into the glass if you happen to be tapering out extra layers of reinforcement, but you do want to avoid it as much as possible in other areas. My impression is that the whole thing could have used an additional filler coat or two. In the top photo you have the dark spots, where you have yet to reach the bottom of the crater with the sander, combined with spots which have already been sanded too deeply and well down into the cloth - all within an inch or two of each other.

If you want to see what cosmetic effect the oversanding will have, you can wipe the area down with a wet sponge. Most of the bad stuff will likely disappear when wet and will do the same once varnished. Strength-wise, it will depend on just how far down into the cloth the sander went in the various spots, but since you aren't likely to contact rocks with every square inch of the hull, you can probably live with the reduced cloth strength. I certainly might be tempted to add one more filler coat before doing any more sanding, and any time you're sanding and start to see a pattern of tiny, evenly-spaced whitish dots showing up, stop sanding that area. That is the top of the weave and further sanding will be cutting into the cloth.

I don't have a sponge but I wiped down with a rag and some warm water. It's not super noticeable although it can be seen if I was finished sanding I would be ok with it aesthetically however the whole hull needs further sanding including in those areas as it's still not very smooth. Im guessing this will further damage the cloth? You can also see I have a couple bubbles by the top of the stem there. I originally did 4 coats of epoxy which I would of thought would be enough but if I have to do one more then so be it although that might not leave me enough epoxy for the inside.

I was thinking of probably doing 120 next. The only reason I was doing 320 was because that's as low as the store had for wet sandpaper.

Re: sanding fiberglass on canoe

That sums it up nicely, and you didn't go far enough into the cloth to make an appreciable difference. I'd fill the low spots with straight epoxy and a putty knife before adding another coat all the way around.

Are you planning to paint, or varnish? If it will be paint you can fill those little low spots and sand them fair then paint.

Re: sanding fiberglass on canoe

Originally Posted by Gib Etheridge

That sums it up nicely, and you didn't go far enough into the cloth to make an appreciable difference. I'd fill the low spots with straight epoxy and a putty knife before adding another coat all the way around.

Are you planning to paint, or varnish? If it will be paint you can fill those little low spots and sand them fair then paint.

Im going to be varnishing in the end. I will apply another coat of epoxy tonight.

I have wiped the hull down with a rag and warm water a few times then wiped it with a tack cloth. As you can see it still has a bit of a milky colour to it. Just want to confirm before I put the epoxy on that layering a new coat of epoxy won't trap that milky white colour in there.

Re: sanding fiberglass on canoe

The best rule of thumb I have ever found was to abandon any thought of predicting the number of filler coats that will be needed. The reason is that it will vary by working temperature, epoxy type, application tool, application (rolling) technique, and even how long that particular batch of resin has been in the roller pan. Instead, you start rolling thin coats, rolled out well, but not too vigorously. Excessive roller pressure or speed creates tiny bubbles suspended in the resin which may not all float out before it sets. Thin coats also have fewer drips and runs. You continue rolling thin coats on until the cloth texture has completely disappeared. Than add one more good one as a sanding cushion.

It is good to give the resin a whole week or so to completely cure before you start sanding, scraping or otherwise smoothing the surface. This is both for you, as skin exposure to resin or resin dust which hasn't cured completely is a health hazard, and for the boat. Excessive agitation of partially cured resin can stress and slightly stretch or bruise the fiberglass, causing tiny white micro-fractures down inside the cloth layer. These are visible and unlike your current surface level problems, they never go away. Using these thin coats (and enough of them) you shouldn't need anything coarser than 80 grit for the first round of sanding.

Re: sanding fiberglass on canoe

Originally Posted by Todd Bradshaw

The best rule of thumb I have ever found was to abandon any thought of predicting the number of filler coats that will be needed. The reason is that it will vary by working temperature, epoxy type, application tool, application (rolling) technique, and even how long that particular batch of resin has been in the roller pan. Instead, you start rolling thin coats, rolled out well, but not too vigorously. Excessive roller pressure or speed creates tiny bubbles suspended in the resin which may not all float out before it sets. Thin coats also have fewer drips and runs. You continue rolling thin coats on until the cloth texture has completely disappeared. Than add one more good one as a sanding cushion.

It is good to give the resin a whole week or so to completely cure before you start sanding, scraping or otherwise smoothing the surface. This is both for you, as skin exposure to resin or resin dust which hasn't cured completely is a health hazard, and for the boat. Excessive agitation of partially cured resin can stress and slightly stretch or bruise the fiberglass, causing tiny white micro-fractures down inside the cloth layer. These are visible and unlike your current surface level problems, they never go away. Using these thin coats (and enough of them) you shouldn't need anything coarser than 80 grit for the first round of sanding.

Thank you for the information. I ended up doing another coat tonight by using a brush to try and fill in the valleys then using a roller to do a complete coat. This is my first time fiberglassing so I knew there would be some errors and I'm ok with that. Unless I sand on Thursday I won't be able to meet the rest of my schedule unless if I skip sanding the outside for now and comeback to it right before varnishing the whole thing. I was hoping to get it done now though as it's still sitting on the strongback and moulds as opposed to later when it would be taken off.

Re: sanding fiberglass on canoe

Your health is more important than your schedule, so if you do decide to sand it green, a good respirator and clothing which keeps the dust off your skin are really critical. You are correct though, that the outside sanding is a lot easier before the hull comes off the forms. A stripper hull is surprisingly floppy before the gunwales and thwarts go in. You actually have to be pretty careful handling it as it comes off the forms and you get it into a position where you can work on the inside. It is entirely possible if not careful to have the thing "fold" on you if it isn't well and evenly supported, and if that happens you will most likely have some serious damage.

Re: sanding fiberglass on canoe

Originally Posted by Gib Etheridge

Did the sanded glass disappear?

The milky white look disapeered. However there is some small white strips which I assume is from the damaged cloth? They aren't really that noticeable. I feel if you didn't know they were there you wouldn't see them.

Safety wise I am good. I have been using a full face respirator with P100/OV filters and covering as much skin as possible for sanding. I am also working in a garage with the door open for some fresh air. My primary concern is ensuring that the canoe is strong enough and to minimize as many aesthetic mistakes as possible since I feel like I've got safety covered.